1. Field of The Invention
The subject invention relates to controlling the contrast in a television receiver.
2. Description of The Related Art
Presently, the contrast of television displays is controlled in four ways. The first two are user controls which the user sets based on his/her own preferences. The last two are built into the television receiver by the manufacturer. These four controls include:
(1) The "Contrast" control on the television receiver varies the gain of the display; increasing the gain increases contrast. However, too much gain can cause clipping of the image highlights. PA1 (2) The "Brightness" control adjusts the DC level of the blackest part of the image. Increasing the brightness control does give the sensation of a brighter image, but at the cost of lower color saturation and lower contrast. PA1 (3) "Gamma correction" is usually built in by the manufacturer. CRT-based displays have a non-linear relationship between the applied signal voltage and the resulting image intensity. Gamma correction seeks to compensate for the distortions caused by such non-linearities by pre-distorting the signal in the opposite direction. This linearization of the image often improves image contrast. It is possible to provide additional low-level contrast by increasing the gamma correction. PA1 (4) "Edge enhancement" steepens the transitions in the signal at detected edges which enhances the visibility of features in an image.
These conventional methods of contrast control are normally applied to the entire image uniformly. Since the dynamic range of displays is limited, it is not possible to simultaneously perceive small variations in image intensity, i.e. small details, in the bright and dark regions simultaneously.